Might as well get Vista, nothing wrong with it unless the OP wants to play old games.
The 8600 is deff not worth an extra £250. Get a shitty card like an 8400, and use the cash saved to buy either an 8800GT or an 4850.
From what moriz said, it seems like if he wants to upgrade video cards on any of these prebuilts (at least if he wants a 4850) he'll also have to upgrade the PSU on these computers we're suggesting to him. I'm having a hard time finding a computer in his price range with a PSU that could handle a 4850.